1. Field of the Disclosure
The present disclosure relates in general to patient monitoring and in particular to oximeter patient monitors capable of determining perfusion index measurements.
2. Description of the Related Art
Oximeter systems providing measurements of a monitored patient have become the standard of care in many patient care settings, including surgical, post surgical, neonatal, general ward, home care, physical training, and the like. In general, oximeter systems accept one or more noninvasive signals from an optical sensor or probe capable of emitting multiple wavelengths of light into a tissue site and capable of detecting light after attenuated by the tissue site. The optical sensor generally outputs intensity signal data. FIG. 1 illustrates a photoplethysmograph intensity signal 100 output by an oximeter sensor. An oximeter does not directly detect absorption, and hence does not directly measure a standard plethysmograph waveform. However, the standard plethysmograph can be derived by observing that the detected intensity signal 100 is merely an out of phase version of an absorption profile known to one of skill in the art. That is, the peak detected intensity 102, generally corresponds to a minimum absorption, and minimum detected intensity 104, generally corresponds to a maximum absorption. Further, a rapid rise in absorption during an inflow phase of the plethysmograph is reflected in a rapid decline 106 in intensity. Likewise, a gradual decline in absorption during the outflow phase of the plethysmograph is reflected in a gradual increase 108 in detected intensity.
FIG. 2 illustrates a flow calculator 200 which receives a processed signal 202 responsive to at least one of the intensity signals output from the sensor. In an embodiment, the flow calculator outputs an indication of blood flow, such as, for example, a perfusion index (PI) 204. In an embodiment, the PI 204 comprises a relative indication of pulse strength at a monitoring site. For example, the PI 204 may be defined as the ratio of the wavelength's (λ) AC signal to the DC signal, or the percentage of pulsatile signal to non-pulsatile signal, according to the following:PI=(λmax−λmin)/λDC 
where λmax is the maximum value, λmin is the minimum value, and λDC is the average value of the signal 202.
Once calculated, the PI 204 may advantageously be displayed in a wide number of ways, including rising LEDs or other display elements, text, graphics, or other visual elements including color, flashing, and the like, trended data, trace data, or the like. FIG. 3A illustrates a display output for an oximeter patient monitor 302 including a textual PI display 304 (shown as “3.25 PI”) ranging from 1.0 to 20. FIG. 3B illustrates a display output for a handheld oximeter patient monitor 322 including a PI bar 324 ranging from, for example, “<0.1%” to “>5%” with steps of “<0.1%,” “0.25%,” “0.5%,” “1%,” “1.25%,” “1.5%,” “1.75%,” “2%,” “3%,” and “>5%.” An artisan will recognize from the disclosure herein that various steps and a wide variety of scalars or other mathematical mapping can be used to make PI numbers more readily understandable to a caregiver. However, using the foregoing scale, the PI bar 324 can be used as a diagnostic tool during low perfusion for the accurate prediction of illness severity, especially in neonates. Moreover, the rate of change in the PI bar 324 can be indicative of blood loss, sleep arousal, sever hypertension, pain management, the presence or absence of drugs, or the like. According to one embodiment, a measurement below about “1.25%” may indicate medical situations in need of caregiver attention, specifically in monitored neonates. Because of the relevance of about “1.25%”, the PI bar 324 may advantageously include level indicia that switch sides of the PI bar 324, thus highlighting any readings below about that threshold. Moreover, behavior of the PI bar 324, as discussed below, may advantageously draw attention to monitored values below such a threshold.
The PI bar 324 may advantageously activate LEDs from a bottom toward a top such that the bar “fills” to a level proportional to the measured value. In one embodiment, the PI bar 324 shows a static value of perfusion for a given time period, such as, for example, one or more pulses. In another embodiment, or functional setting, the PI bar 324 may advantageously pulse with a pulse rate, may hold the last reading and optionally fade until the next reading, may indicate historical readings through colors or fades, traces or the like. Additionally, the PI bar 324 may advantageously change colors, flash, increasingly flash, or the like to indicate worsening measured values of perfusion.
As discussed above, the monitors 302, 322 may include output functionality that outputs, for example, trend perfusion data, such that a caregiver can monitor measured values of perfusion over time. Alternatively or additionally, the monitors 302, 322 may display historical trace data on an appropriate display indicating the measured values of perfusion over time. In an embodiment, the trend data is uploaded to an external computing device through, for example, a multipurpose sensor connector or other input output systems such as USB, serial or parallel ports, computing networks, or the like. Additional information regarding the calculation and use of PI is disclosed in the '065 patent, assigned to Masimo Corporation (“Masimo”) of Irvine, Calif., and incorporated by reference herein.